1. Field of the Invention
The present invention relates to a power module, and more specifically to an intelligent power module in which a power semiconductor element and a peripheral control circuit for controlling the power semiconductor element are combined.
2. Description of the Related Art
There is a tendency that a power module is constructed as an intelligent power module in which a power semiconductor element and peripheral control circuits are combined, as in an inverter. The heat radiating characteristics are regarded to be an important factor in a power module. A DBC substrate has a structure in which a ceramic plate is sandwiched between copper plates, and is expensive. For this reason, in low to middle power modules, an insulated metal substrate (IMS) is used in many cases. The IMS has a structure in which an insulation layer is formed on a metal base made of, for example, aluminum, and a desired conducting pattern is formed on the insulation layer.
FIG. 1 is a cross section of a conventional power module in which an IMS is used. The IMS consists of a metal base 301, a high heat radiating insulation layer 302, and a conductor pattern 303 formed on the high heat radiating insulation layer 302. The power semiconductor element 31 is soldered onto a heat spreader 32 fixed on the conductor pattern 303 of the IMS 30. Semiconductor elements 33 such as transistors and capacitors, which serve as control circuits for the power semiconductor element 31, are connected to the conductor pattern 303 on the IMS 30.
With the above-described structure in which both a power semiconductor element portion and a control circuit are provided on a common plate of IMS 30. In order to avoid this, it is necessary to maintain a predetermined distance or more between wiring regions of these portions. Therefore, the flexibility in designing of the IMS 30 is low. For example, in some cases, it is difficult to match the terminals of the IMS 30 with the predetermined position of the terminals of the enclosure. Further, the control circuit is complicated, and it is also necessary in the power module to mount elements at a high density by using fine wiring materials.
As shown in FIG. 2, the IMS 30 is formed to have a partial double-layer structure in which only the control circuit portion is made into a double-layer. The double-layer structure includes a high heat radiating insulation layer 302b, which is the second layer, formed on a conductor pattern 303a, which is the first layer, and a conductor pattern 303b, which is the second layer, formed on the layer 302b. In the IMS 30 having such a partial double-layer structure, both wiring regions for the power semiconductor element portion and the control circuit portion are insulated by the high heat radiating insulation layer 302b.
However, since the IMS 30, which is partially double-layered, has a very complex structure, and is prepared by a very complex production process, the production cost for the IMS 30 is very much increased, and it should not be applied to inverters for household appliances or other general items. In the case where only the output of the power semiconductor element is varied, it is necessary to redesign the entire IMS 30 including the control circuit portions and the like, increasing the cost for the developing and consuming a great deal of time.